The present invention relates to a method and a system for monitoring the operation of a sensor. Also, the present invention relates to a method and a system for diagnosing malfunctioning systems by comparing measured values with reference values.
A modern vehicle is normally equipped with multiple systems for monitoring the actual condition of different components within the vehicle. Such vehicle components may represent relatively complex assemblies within the vehicle, such as the complete internal combustion engine or the braking system, or they may represent relatively simple elements such as individual sensors or actuators. Each component contributes to the overall performance of the vehicle and it is desirable to retrieve data corresponding to the actual operation of such components in order to detect malfunctioning and to predict maintenance.
For monitoring the actual condition of a vehicle component it is necessary to determine parameter values being associated with the actual operation of the particular vehicle component. By comparing the determined parameter values with reference values it is possible to evaluate the actual condition and to decide if the vehicle component is operating normally or not.
In WO2008/140381 a method for diagnosing malfunctioning components in a vehicle is described. In this document component-related signals describing the status of different components are used for determining a significant relation between them. The determination whether a relation is significant or not is performed by comparing the compatible relations between the different components. In case a significant relation is detected, this significant relation is compared between components, or compared for single components over time, and used as a basis on which a decision on a necessity for providing maintenance and/or repair to an individual component can be reached.
In some cases, like e.g. a single regulating component being monitored by a vehicle sensor, such particular system may be associated with a single component-related signal only, i.e. the sensor signal. Hence, it is not necessary to determine a significant relation between different component-related signals. Instead the single component-related signal may give information representing the actual condition of the vehicle component by comparing the signal parameters with reference values. Malfunction may thus be detected if the actual signal parameters are deviating from the reference values. For the vehicle system described above it may however be hard to detect a malfunction of the sensor itself. Typical sensor errors include drifting and a deteriorated step response, and should any of these errors start to occur it may no longer be possible to detect malfunctioning of the vehicle component accurately.
Therefore there is a need for an improved method for monitoring the operation of a vehicle sensor, as well as an improved method for determining the condition of a vehicle system.
It is desirable to provide an improved method and system for determining if a sensor is operating normally or not.
According to a first aspect, a method comprises the step of representing the operation of the sensor by: i) storing a plurality of data values, each data value corresponding to the sensor output signal, wherein said step is performed during a time period such that said data values are distributed over a range of possible data values, ii) defining a plurality of discrete intervals within said range of possible data values; and iii) calculating the frequency of the data values within each interval thus forming a sensor representation. Further, the method comprises the steps of:
receiving at least one reference sensor representation; and
comparing said sensor representation with said at least one reference sensor representation.
In a preferred embodiment, the system sensor is a NOx sensor of a vehicle. The method is preferably implemented for a NOx sensor due to the fact that i) the NOx sensor is a critical component for determining the status of the aftertreatment system, whereby failure may lead to increased environmental damage, and ii) the NOx sensor has a limited number of failure modes whereby the proposed method may rapidly detect malfunction in a very efficient manner.
The step of storing a plurality of data values may be performed for a plurality of different operation modes of a system to which said sensor is connected. Since the data values will most likely be different depending on different operation modes, the sensor representation will consequently have an improved statistical significance.
The reference sensor representation may include the frequency of reference data values within discrete reference intervals, said discrete reference intervals corresponding to the discrete intervals of the sensor representation.
Hence, it will be possible to compare the sensor representation with the reference representation in a simple and efficient manner.
The step of comparing said sensor representation with said reference sensor representation may further comprise the step of normalizing said representations, whereby comparison between the representations is further facilitated.
In an embodiment, the step of comparing said sensor representation with said reference sensor representation may further comprise: calculating a function representing the difference between the sensor representation and the reference sensor representation, and classifying said function according to a predetermined classification scheme. The monitoring of the sensor may thus result in a single classification, whereby the particular classification may be associated with robust information giving a clear indication of the operation of the sensor.
The classification scheme may comprise a plurality of reference categories, wherein each reference category is associated with a specific operation of the sensor. For example, this allows the reference categories to include one category corresponding to a sensor operating normally, and at least one category corresponding to a sensor operating non-normally. It may thus be possible to determine a faulty sensor in a simple manner.
The reference categories may further include one category corresponding to a sensor operating normally in a system operating normally, and at least one category corresponding to a sensor operating normally in a system operating non-normally. Hence, the method allows for system monitoring, whereby the sensor representation may not only indicate the actual status of the sensor, but also the actual status of an associated system. Should the system be a complex system for which several different fault types is possible, it may be possible to expand the bundle of reference representations to also cover different fault types for the system. Hence, a sensor representation may thus be used to identify the most similar reference representation, and to determine the actual fault type associated with the identified reference representation.
In a preferred embodiment, the system is an exhaust aftertreatment system of a vehicle. Hence, monitoring the operation of the NOx sensor may not only provide useful information regarding the actual status of the sensor itself, but also useful information regarding the operational status of the urea injection system without any direct measurements on the urea injection system.
The function representing the difference between the sensor representation and the reference sensor representation may be a scalar, whereby it is possible to provide an improved, and very robust, classification by setting specific intervals of the scalar.
In an embodiment, the method may be repeated for a plurality of times resulting in a set of functions, and further comprising the step of evaluating said set of functions for predicting malfunctioning of the sensor. It is thus possible to use the method for analyzing the behavior of the sensor and/or the associated system over time in order to predict future behavior. Hence it may be possible to determine a specific time in the future for service or maintenance thus reducing the risk for unplanned down time of the system.
The reference sensor representation may be a previous sensor representation for said sensor. The method may thus continuously store data values, whereby previously stored data values may form a reference representation.
In other embodiments, the reference sensor representation may correspond to one or several sensor representations for one or more other sensors. These other sensors may form part of different systems, e.g. different vehicles, and in a preferred embodiment the reference representation is created from data values fetched from a fleet of vehicles.
According to a second aspect, a sensor system comprises a controller for monitoring the operation of the sensor, said controller comprising a first memory configured to store a representation of the sensor operation, said sensor representation including a plurality of data values being distributed over a range of possible data values, each data value corresponding to the sensor output signal, and wherein said representation holds information relating to a frequency distribution of said data values within a plurality of discrete intervals. The controller further comprises a second memory configured to store at least one reference sensor representation, and a comparing unit configured to compare said sensor representation with said at least one reference sensor representation.
The sensor system may further comprise a communication unit configured to receive said sensor representation and said reference sensor representation from said first memory and said second memory; and to transmit said sensor representation and said reference sensor representation to said comparing unit.
The first memory may be arranged on board a vehicle, and said second memory and said comparing unit may be arranged off board a vehicle.